• Energy Research

Abstract Small-scale deformation bands in Penrith Sandstone are used to assess the extent to which these features can act as effective mini-traps and contribute to secure CO2 geological storage. A comprehensive set of simulation scenarios is applied to one conjugate set of deformation bands and also to clusters of deformation bands, to evaluate the effects of i) deformation band density; ii) the contrast in host rock/deformation band permeability; and iii) deformation band geometry, orientation and distribution on fluid movement and its significance for CO2 storage capacity and security. The findings of this study show that one conjugate set of deformation bands can improve CO2 storage security, depending upon the plunge angle of the hinge. It has also been demonstrated that a high contrast in permeability (at least three orders of magnitude) is necessary for the CO2 to be effectively trapped by the deformation bands. It is shown that the highest number of bands observed and modelled for Penrith Sandstone outcrop, with three orders of magnitude permeability contrast, is a configuration that can contribute to the secure storage of CO2 without causing an injectivity issue. This study shows that storage security is not only controlled by the contrast in permeability, but also by the permeability of the host rock. Furthermore, some geometries may contribute to storage security, while others may compromise it. To improve storage capacity and security for the type of reservoir studied herein, the results demonstrate the importance of accounting for the optimum injection rate and well placement.

Les puits forés dans des sites de stockage de carbone pourraient être convertis en voies de fuite potentielles en présence de fluides contenant du CO2 et sous l'impact des changements survenant dans le stress souterrain. Pour tester cette hypothèse, dans cette étude, le comportement du ciment de puits de pétrole de classe G en contact avec le CO2 supercritique a été étudié. Les noyaux de ciment ont été durcis sous eau saturée de chaux pendant 28 jours à une température de 20 °C et sous pression atmosphérique. Par la suite, ils ont été exposés à du CO2 supercritique sous une pression de 20 MPa et à une température de 90 °C pendant 30 jours. La profondeur de pénétration du front de carbonatation et le changement des propriétés poromécaniques du noyau de ciment ont été mesurés en fonction du temps. Un exercice de modélisation numérique a également été mené pour simuler l'altération dans les carottes de ciment. Les résultats présentés dans cette étude montrent que la précipitation des carbonates de calcium réduit la porosité au sein des couches les plus externes des noyaux de ciment. Ce phénomène déplace la classe de taille de pore principale vers des tailles plus petites. Contrairement aux attentes, la réduction de la porosité n'améliore pas la résistance globale des échantillons de ciment. La réduction observée de la résistance des échantillons de ciment pourrait être associée soit à la structure amorphe des carbonates précipités, soit à la faible liaison entre eux et les parois solides des pores et à la forte dégradation des hydrates de silicate de calcium. Los pozos perforados en los sitios de almacenamiento de carbono podrían convertirse en posibles vías de fuga en presencia de fluidos que contengan CO2 y bajo el impacto de los cambios que ocurren en la tensión subterránea. Para probar esta hipótesis, en este estudio, se ha investigado el comportamiento del cemento de pozo petrolífero Clase G en contacto con CO2 supercrítico. Los núcleos de cemento se curaron bajo agua saturada de cal durante 28 días a una temperatura de 20 °C y a presión atmosférica. Posteriormente, se expusieron a CO2 supercrítico a una presión de 20 MPa y a una temperatura de 90 ºC durante 30 días. La profundidad de penetración del frente de carbonatación y el cambio en las propiedades poromecánicas del núcleo de cemento se midieron frente al tiempo. También se ha realizado un ejercicio de modelado numérico para simular la alteración dentro de los núcleos de cemento. Los resultados presentados en este estudio muestran que la precipitación de carbonatos de calcio reduce la porosidad dentro de las capas más externas de los núcleos de cemento. Este fenómeno desplaza la clase de tamaño de poro principal hacia tamaños más pequeños. En contraste con las expectativas, la reducción de la porosidad no mejora la resistencia general de las muestras de cemento. La reducción observada en la resistencia de las muestras de cemento podría estar asociada con la estructura amorfa de los carbonatos precipitados o la débil unión entre ellos y las paredes sólidas de los poros y la alta degradación de los hidratos de silicato de calcio. Wells drilled in carbon storage sites could be converted to potential leakage pathways in the presence of CO2-bearing fluids and under the impact of the changes occurring in underground stress. To test this hypothesis, in this study, the behavior of Class G oil well cement in contact with supercritical CO2 has been investigated. The cement cores were cured under lime-saturated water for 28 days at a temperature of 20 ∘C and under atmospheric pressure. Subsequently, they were exposed to supercritical CO2 under a pressure of 20 MPa and at a temperature of 90 ∘C for 30 days. The penetration depth of the carbonation front and the change in the poromechanical properties of the cement core were measured against time. A numerical modeling exercise has also been conducted to simulate the alteration within the cement cores. The results presented in this study show that the precipitation of calcium carbonates reduces the porosity within the outermost layers of the cement cores. This phenomenon shifts the main pore size class towards smaller sizes. In contrast to expectations, the reduction in porosity does not improve the overall strength of the cement specimens. The observed reduction in the strength of the cement specimens might be associated with either the amorphous structure of the precipitated carbonates or the weak bonding between them and the solid walls of the pores and the high degradation of calcium silicate hydrates. يمكن تحويل الآبار المحفورة في مواقع تخزين الكربون إلى مسارات تسرب محتملة في وجود سوائل حاملة لثاني أكسيد الكربون وتحت تأثير التغيرات التي تحدث في الإجهاد تحت الأرض. لاختبار هذه الفرضية، في هذه الدراسة، تم التحقيق في سلوك أسمنت بئر النفط من الفئة ز في اتصال مع ثاني أكسيد الكربون فوق الحرج. تمت معالجة قلوب الأسمنت تحت الماء المشبع بالجير لمدة 28 يومًا عند درجة حرارة 20 درجةمئوية وتحت الضغط الجوي. بعد ذلك، تعرضوا لثاني أكسيد الكربون فوق الحرج تحت ضغط 20 ميجا باسكال وعند درجة حرارة 90 درجةمئوية لمدة 30 يومًا. تم قياس عمق اختراق جبهة الكربنة والتغير في الخصائص الميكانيكية البورومية لقلب الأسمنت مع مرور الوقت. كما تم إجراء تمرين نمذجة رقمية لمحاكاة التغيير داخل النوى الأسمنتية. تظهر النتائج المقدمة في هذه الدراسة أن ترسيب كربونات الكالسيوم يقلل من المسامية داخل الطبقات الخارجية من النوى الأسمنتية. تحول هذه الظاهرة فئة حجم المسام الرئيسية نحو أحجام أصغر. وعلى النقيض من التوقعات، فإن انخفاض المسامية لا يحسن القوة الإجمالية لعينات الأسمنت. قد يرتبط الانخفاض الملحوظ في قوة عينات الأسمنت إما بالبنية غير المتبلورة للكربونات المترسبة أو الترابط الضعيف بينها وبين الجدران الصلبة للمسام والتحلل العالي لهيدرات سيليكات الكالسيوم.

The UK plans to bring all greenhouse gas emissions to net-zero by 2050. Carbon capture and storage (CCS), an important strategy to reduce global CO2 emissions, is one of the critical objectives of this UK net-zero plan. Among the possible storage site options, saline aquifers are one of the most promising candidates for long-term CO2 sequestrations. Despite its promising potential, few studies have been conducted on the CO2 storage process in the Bunter Closure 36 model located off the eastern shore of the UK. Located amid a number of oil fields, Bunter is one of the primary candidates for CO2 storage in the UK, with plans to store more than 280 Mt of CO2 from injections starting in 2027. As saline aquifers are usually sparsely drilled with minimal dynamic data, any model is subject to a level of uncertainty. This is the first study on the impact of the model and fluid uncertainties on the CO2 storage process in Bunter. This study attempted to fully accommodate the uncertainty space on Bunter by performing twenty thousand forward simulations using a vertical equilibrium-based simulator. The joint impact of five uncertain parameters using data-driven models was analysed. The results of this work will improve our understanding of the carbon storage process in the Bunter model before the injection phase is initiated. Due to the complexity of the model, it is not recommended to make a general statement about the influence of a single variable on CO2 plume migration in the Bunter model. The reservoir temperature was shown to have the most impact on the plume dynamics (overall importance of 41%), followed by pressure (21%), permeability (17%), elevation (13%), and porosity (8%), respectively. The results also showed that a lower temperature and higher pressure in the Bunter reservoir condition would result in a higher density and, consequently, a higher structural capacity.

Abstract Saline aquifers constitute the most abundant geological storage option for Carbon Capture and Storage (CCS) projects. When injected in the aquifer, due to its lower density in comparison to the in-situ brine, the free phase CO2 tends to migrate upwards. This vertical migration is generally tens of metres depending on the reservoir thickness, despite the plume migration distance in the horizontal direction which could be over hundreds of kilometres (depending on the time horizon, reservoir characteristics, trapping mechanisms involved, etc.). In many situations, the plume ends up as a separate region below a sealing barrier. This large aspect ratio between the plume migration in the horizontal and vertical directions would potentially validate the use of vertical equilibrium (VE) models in CO2 storage studies. In other words, when phase segregation occurs rapidly compared to the time scale studied, vertical equilibrium can be assumed, allowing for the use of specially adapted models. In the VE model, the equilibrium between brine and CO2 is pre-assumed at all times. Under this assumption, the injected CO2 plume flow in 3D can be approximated in terms of its thickness in order to obtain a 2D simulation model, which consequently decreases the computational costs. The time by which phase segregation occurs depends on the aquifer thickness, aquifer permeability, fluid properties, etc. However, the CO2 and in-situ brine are separated considerably fast and form two separate layers, in comparison to the time period for lateral migration. The CO2lab module of the Matlab Reservoir Simulation Toolbox (MRST) used in this work, is a set of open source simulation and workflow tools to study the long-term, large-scale storage of CO2. We employed the VE tool in MRST−CO2lab (MVE) to study the effect of caprock morphology on the CO2 migration. The results have been compared with a number of simulators including ECLIPSE-black-oil (E100), ECLIPSE-compositional (E300) and ECLIPSE-VE (EVE) models and the differences between the approaches are analysed and discussed in detail. In particular, we focused on the impact of caprock morphology and aquifer top-surface slope on the CO2 structural and dissolution trapping mechanisms and plume migration. The results indicated a good agreement for the ultimate plume shapes in all the models. However, the amount of dissolved CO2 in the brine was different.

Abstract Numerical models of geologic carbon sequestration (GCS) in saline aquifers use multiphase fluid flow-characteristic curves (relative permeability and capillary pressure) to represent the interactions of the non-wetting CO2 and the wetting brine. Relative permeability data for many sedimentary formations is very scarce, resulting in the utilisation of mathematical correlations to generate the fluid flow characteristics in these formations. The flow models are essential for the prediction of CO2 storage capacity and trapping mechanisms in the geological media. The observation of pressure dissipation across the storage and sealing formations is relevant for storage capacity and geomechanical analysis during CO2 injection. This paper evaluates the relevance of representing relative permeability variations in the sealing formation when modelling geological CO2 sequestration processes. Here we concentrate on gradational changes in the lower part of the caprock, particularly how they affect pressure evolution within the entire sealing formation when duly represented by relative permeability functions. The results demonstrate the importance of accounting for pore size variations in the mathematical model adopted to generate the characteristic curves for GCS analysis. Gradational changes at the base of the caprock influence the magnitude of pressure that propagates vertically into the caprock from the aquifer, especially at the critical zone (i.e. the region overlying the CO2 plume accumulating at the reservoir-seal interface). A higher degree of overpressure and CO2 storage capacity was observed at the base of caprocks that showed gradation. These results illustrate the need to obtain reliable relative permeability functions for GCS, beyond just permeability and porosity data. The study provides a formative principle for geomechanical simulations that study the possibility of pressure-induced caprock failure during CO2 sequestration.

Abstract Although the application of multi-phase flow meters has recently increased, the production of individual wells in many fields is still monitored by occasional flow tests using test separators. In the absence of flow measurement data during the time interval between two consecutive flow tests, the flow rates of wells are typically estimated using allocation techniques. As the flow rates, however, do not remain the same over the time between the tests, there is typically a large uncertainty associated with the allocated values. In this research, the effect of the frequency of flow tests on the estimated total production of wells, allocation, and hydrocarbon accounting has been investigated. Allocation calculations have been undertaken for three different cases using actual and simulated production data based on one to four flow tests per month. Allocation errors for each case have subsequently been obtained. The results show that for all the investigated cases, the average allocation error decreased when the number of flow tests per month increased. The sharpest error reduction has been observed when the frequency of the tests increased from one to two times per month. It reduced the allocation error for the three investigated cases by 0.43%, 0.45%, and 1.11% which are equivalent to $18.2M (million), $18.9M, and $46.8M reduction in the yearly cost of the allocation error for the respective cases. The reductions in the allocation error cost for the three cases were $27M, $29M, and $80M, respectively, when the flow tests have been undertaken weekly instead of monthly.

Abstract In this proposed CO 2 injection system, brine is extracted from the target storage aquifer by means of a lateral horizontal completion located near the top of the formation. It should be noted that the brine is not lifted to the surface. An Electrical Submersible Pump (ESP) is used to extract the brine and boost its pressure, before it is mixed with CO 2 that is injected down the vertical section of the well. The mixing takes place in the vertical section of the well below the upper lateral. The CO 2 –brine mix is then injected into the same formation through a lower lateral. A down-hole tool would be used to maximise agitation and contact area between CO 2 and brine in the vertical mixing section of the well, which may be tens to hundreds of metres long, depending on the thickness of the formation. The advantages of this method are that there is little overall pressure increase, because CO 2 is mixed with brine extracted from the formation, and also the extracted brine is already at high pressure when it is mixed with the CO 2 , greatly increasing the solubility of CO 2 and reducing the volume of brine required. Energy is not expended lifting the brine to surface nor is there any concern about handling large volumes of acidic brine in the surface equipment. In this study, in addition to the concept of the down-hole mixing (DHM) method which is presented, the application of the DHM method in a hypothetical storage site (Lincolnshire—Smith et al., 2012) is also examined. The calculations are performed to identify the optimum rates of water extraction and injection of dissolved CO 2 in brine.

Abstract When investigating the storage of CO 2 in deep saline formations, many studies assume a smooth, abrupt interface between the storage and the sealing formations. Typically, though, the surface is irregular, due to sedimentological and stratigraphic effects or structural deformation. In this study, the area where the CO 2 migrates beneath the caprock is investigated. A set of numerical simulations were conducted to investigate the impacts of various factors on CO 2 storage, such as top morphology, tilt, k v /k h ratio and the presence of a transition zone, where there is a gradational change from storage formation to caprock. In the models tested, the k v /k h ratio was most important during the injection period, but after injection ceased, the tilt was more important. The amplitude of the ridges, which were used to represent the top morphology, did not have a large effect but, as expected hindered or encouraged migration depending on whether they were perpendicular or parallel to the tilt. A transition zone can increase the security of storage by lessening the amount of CO 2 accumulating underneath the caprock. Therefore it is important to characterise the interface in terms of the size of irregularities and also in terms of the existence of any transition zone. The latter has not been addressed in previous works. A simple formula was derived to predict the limiting tilt for trapping to occur in models with a sinusoidal interface with wavelength, λ, and amplitude, A. Although this is a simplified approach, it provides a means of assessing whether the topography of the top surface will give rise to significant trapping or not.

The implementation of CO2 storage in sub-surface sedimentary formations can involve decision making using relevant numerical modelling. These models are often represented by 2D or 3D grids that show an abrupt boundary between the reservoir and the seal lithologies. However, in an actual geological formation, an abrupt contact does not always exist at the interface between distinct clastic lithologies such as sandstone and shale. This article presents a numerical investigation of the effect of sediment-size variation on CO2 transport processes in saline aquifers. Using the Triassic Bunter Sandstone Formation (BSF) of the Southern North Sea (SNS), this study investigates the impact a gradation change at the reservoir-seal interface on CO2 sequestration. This is of great interest due to the importance of enhanced geological detail in reservoir models used to predict CO2 plume migration and the integrity of trapping mechanisms within the storage formation. The simplified strategy was to apply the Van Genutchen formulation to establish constitutive relationships for pore geometric properties, which include capillary pressure (Pc) and relative permeability (kr), as a function of brine saturation in the porous media. The results show that the existence of sediment gradation at the reservoir-seal interface and within the reservoir has an important effect on CO2 migration and pressure diffusion in the formation. The modelling exercise shows that these features can lead to an increase in residual gas trapping in the reservoir and localised pore pressures at the caprock’s injection point.